One sticking point, according to the Washington Post, is Hagel's position on Israeli security. Hagel complained of the "Jewish lobby" and its influence in Washington, D.C., and once commented that he was Nebraska's senator, not Israel’s, according to Slate.

But as Harretz pointed out, Hagel wrote in his 2008 book, "America: Our Next Chapter": "There will always be a special and historic bond with Israel, exemplified by our continued commitment to Israel’s defense."

However, Hagel followed that statement with, "But this commitment cannot be at the expense of our Arab and Muslim relationships.”

This might be why Graham said Hagel would be "the most antagonistic secretary of defense toward the State of Israel in our nation's history."

Republicans also aren't too keen on Hagel's stance against the war in Iraq. As a Senate Foreign Relations Committee member, Hagel broke with his party and criticized the war; according to Slate's Fred Kaplan, Hagel opposed both invading Iraq and the 2007 troop surge, positions the Republicans mostly favored.

Hagel once said:

"How many of us really know and understand much about Iraq, the country, the history, the people, the role in the Arab world? I approach the issue of post-Saddam Iraq and the future of democracy and stability in the Middle East with more caution, realism and a bit more humility."

According to multiple news outlets, Hagel has said America's defense budget is bloated, and voted against some Iran sanctions—actions many Republicans, and perhaps some Democrats, will not take kindly.

The left has also criticized Hagel. According to NBC News, gay rights groups protested comments Hegel made in 1998 that criticized President Bill Clinton's nomination of a U.S. ambassador who happened to he gay. Hagel later apologized.

Hagel, who is currently the Edmund A. Walsh School Professor of Foreign Service at Georgetown University, was twice awarded the Purple Heart in Vietnam, and served as Nebraska's senator for two terms, ending in 2009.

Republican Senate Minority Leader Mitch McConnell of Kentucky admitted on ABC’s “This Week” that Hagel had been "outspoken in foreign policy and defense over the years." He added that Hagel "ought to be given a fair hearing, like any other nominee.”